1. Field of the Invention
The present invention generally relates to a spinal fixation system for use in the immobilization of a sequence of spinal bones. More particularly, the present invention relates to a spinal fixation system which allow the implantation of the pedicle screws at the best anatomic location and orientation with flexibility of screw placement and alignment; easy and simple connection between the rod and the screws without application of excessive force to the spine and/or implants during assembly; the top-tightening mechanism to lock the assembly in place with better visibility and access to the spine and implants; a rigid segmental fixation for enhanced maintenance of correction and solid fusion process; and a low profile of the assembly.
2. Description of the Prior art
The human spine is a complex columnar structure of vertebral bone and connective tissues. The vertebrae, discs, and ligaments are intricately arranged, and the complex interaction amongst these structures provides flexibility for motion, spinal cord protection and distribution of body forces. In the diseased or injured state, this delicate equilibriums disturbed and results in spinal pathologies. In many cases, the spinal disorders can be treated by a conservative nonsurgical methods, such as medication, exercise and physical therapies. However, some spinal disorders, such as degenerative instability, deformity, trauma, and tumors, require a surgical intervention to treat pain induced by nerve root compression and unstable invertebral joints.
The surgical procedures for the thoracolumbar spine involves the dissection of soft tissues and often the removal of load bearing structures, such as vertebral bone and discs to decompress the neural elements. Such decompression procedures lead to spinal instability and it is often necessary to fuse spinal segment to restore the stability. Internal fixation with instrumentation accompanies surgical fusion to augment the bony fusion by achieving temporary but rigid fixation that provides a stable environment for bone fusion as well as a maintenance of corrected alignment of the spine.
A variety of internal spinal fixation systems have been developed and used in spine surgery to achieve such a rigid fixation by implanting artificial assemblies in or on the spine. Spinal implants can be classified as anterior or posterior instrumentation systems based upon the implanting location. Anterior implants are coupled to the anterior portion of the spine. The use of posterior implants using pedicle screws coupled by longitudinal rods has become more popular because of their capability of achieving a rigid fixation. Such posterior implants generally comprise pairs of rods, which are aligned along the longitudinal axis of the spine, and which are then attached to the spinal column by screws which are inserted through the pedicles into respective vertebral bodies.
The surgical procedure to achieve a posterior fixation using pedicle screws and connecting rods generally includes the insertion of screws into pedicles in a predetermined angle and depth, temporary coupling of the rods to the screws, the proper correction of spinal curve, and the secure connection of rods to the screws for rigid fixation. A considerable difficulty identified in this surgical procedure is associated with the coupling of a rod to a plurality of screws that are not well aligned in general because the angle and depth of the screw insertion should be determined by patient' anatomical and pathological conditions that may vary among spinal levels as well as among patients. It has been identified that attempts for addressing such a difficulty result in the application of unnecessarily excessive loads to the spinal column via the pedicle screws and the increased operation time, which are known to cause many complications associated with surgery.
For successful posterior spinal instrumentation, it is essential to securely mount the screws and the rods on the spinal column without applying unnecessary forces to the spine and the implants which may cause an acute or fatigue failure of the surgical construct. A variety of attempts have been made to address this issue. Such attempts can be classified into two methods. The first method is the use of a connector comprising a hole to adapt a rod at one end and an oblong hole used for coupling with screws at the other end (ISOLA.RTM. Spinal System, Acromed Corporation, Clevelan, Ohio). This connector permits some freedom with respect to the distance between the rod and screw axes by the use of an oblong hole, but not with respect to angulation of the screw and the coupling element. A variety of washers with a declined surface at various angles are used to obtain some freedom with respect to angulation in coupling the angulated screws and the rod. However, the selection and use of various washers in as proper manner is a tedious manipulation of many small parts during surgery in which there should be extreme limitations in terms of time and space. The use of washers also elevates the profile of the assembly, which causes patients' discomfort and often results in additional surgery for removal of the implants. The other method is the use of polyaxial screws that permits freedom with respect to angulation of the screw. The use of polyaxial screws, however, generally has difficulties in coupling the screw with the rod located apart from the screw head. An example of a polyaxial pedicle screw having a through bar clamp locking mechanism is disclosed in U.S. Pat. No. 5,961,518 to Errico et al., issued Oct. 5, 1999. Such polyaxial pedicle includes a pedicle screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception, and which comprises a reduced number of elements and thus correspondingly provides for expeditious implantation. In this invention, however, the screw and rod assembly should be locked using a nut oriented sideways (i.e., the tightening nut faces the lateral side of the patient during surgery), which is very inconvenient to perform in a very limited lateral operating space. In fact, most surgeons prefer the top-tightening mechanism (i.e., the axis of tightening member faces the posterior side of the patient) because the top-tightening mechanism provides better visibility and access than the side-tightening mechanism. Polyaxial couplings of the screw and the rod used in previous inventions are also not likely to provide either sufficient joint-locking power to prevent the slippage and rotation of the screw with respect to the rod or the polyaxial freedom of implantation angulation. The sufficient locking power in couplings is required to achieve a rigid fixation to provide a stable environment for successful bony fusion and also to maintain the correction until solid fusion occurs. Freedom i controlling both angulation and distance between the screw and the rod is required not only to reduce the surgery time but also to prevent the application of unnecessary stresses on the implant and the spine which may cause a failure in surgery.